Traditionally, preserved foods have been packed in metal or glass containers. However, as a result of technology developed in recent years, it has become possible to pack preserved, i.e. sterile, foods in packaging containers constructed from paperboard laminate, the paperboard laminate being given specific properties with the result that it withstands a sterilising retorting after filling of the food. For example, special qualities of polypropylene are employed for the inner and outer liquid-tight layers of the paperboard laminate. Specific dimensions are also employed for other layers in the packaging laminate as well as choice of qualities which differ from corresponding dimensions and choice of aseptic applications.
From, for example, a flat-folded tubular packaging blank of the prior art known packaging laminate, retortable packaging containers are produced in that the packaging blank is first raised to an open, tubular packaging carton which is sealed at its one end by fold forming and thermosealing of continuous foldable end panels of the packaging carton for forming a substantially planar bottom seal. The packaging carton provided with the bottom is filled with the relevant contents, for example food, through its open end which is thereafter closed and sealed by additional fold forming and thermosealing of corresponding end panels of the packaging carton for the formation of a substantially planar top seal. Naturally, the top seal of the packaging carton may instead be formed first, in which event filling instead takes place via the bottom. The filled and sealed, normally parallelepipedic packaging container is then ready for a heat treatment in order to impart extended shelf-life to the packed contents or food in its unopened packaging container, for example a shelf-life of at least 6 months, often even longer, such as at least 12 or 18 months.
A heat treatment intended to extend shelf-life (retorting) may suitably be put into effect in the manner and under the conditions which are described in international Patent Application carrying publication number WO98/16431. In such instance, the packaging container is placed in a retort and heated therein with the aid of a first gaseous medium flowing in contact with the outer walls of the packaging container, for example hot steam, to a temperature in general within the range of between 70 and 130° C. After a predetermined hold-time at the selected temperature, the supply of the gaseous medium is discontinued. The packaging container is cooled by means of a second circulating gaseous medium, for example cold air, and finally by means of a circulating liquid medium, for example cold water. The cooled retorted packaging container is thereafter removed from the retort for storage, transport and/or other handling.
One variation of such a retorting is so-called hot fill which is particularly utilised for acidic products below a pH of 4.6. In such instance, a filling of the product takes place with at least one component thereof at least 80° C. but below 100° C., entailing a pasteurisation, whereafter retorting is not required because of the fact that the food product is acidic. At least some component of the food product is thus heated to at least 80° C. but below 100° C. already before being filled into the packaging container. In order to ensure sterility, the closed and sealed packaging container is held warm during a predetermined period of time after the hot fill, suitably without the temperature of the packaging container and its food contents being allowed to fall appreciably between hot fill and holding. During the period of holding the food and container, the temperature is maintained at least 80° C. but below 100° C. by contact with hot water flowing over the outer walls of the packaging container. After having been held, the packaging container and its food contents are cooled by means of a coolant which may be cold water.
Within the industry, use is made today of the same type of paperboard laminate for hot fill process of acidic foods as for packaging containers which are retorted. Since no retorting is carried out in connection with a hot fill process, such a paperboard laminate is, however, in actual fact overdimensioned and, consequently, unnecessarily expensive for the hot fill process proper. However, if a simpler paperboard laminate is employed, such as a paperboard laminate with inner and outer liquid-tight layers of polyethylene-based polymer and simpler/more economical qualities also in other layers, the packaging container will become soft and sloppy during the heat holding phase.
It has also proved that problems may arise in connection with the packing of acidic food products in packaging containers formed from paperboard laminate which is particularly adapted for retorting. Such acidic products may be food products containing or packed in acetic acid or other acid, or food products which, during storage, release free fatty acids, which may in particular be the case for food products of the tomato variety. Hereafter, such compounds will be designated acidic compounds regardless of whether they relate to acids such as acetic acid or free fatty acids which have been released from the food product. These acidic compounds have proved to be capable of penetrating the inside of the packaging laminate, but not the aluminium foil. In such instance, they accumulate in steadily increasing concentration adjacent the aluminium foil and gradually, during the long shelf-life which the packaging container nevertheless displays, have a negative effect on the binding or adhesion between the aluminium foil and the liquid-tight inside coating based on polypropylene. In the worst case scenario, there is a risk that the adhesion releases totally so that the inside coating based on polypropylene forms a loose bag inside the packaging container.